Avoid an expensive attachment counting query on the post editing screen.

Merges [28191], [28194] to the 3.9 branch.

props johnbillion.
fixes #27985.

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git-svn-id: http://core.svn.wordpress.org/branches/3.9@28089 1a063a9b-81f0-0310-95a4-ce76da25c4cd